1. Field of the Invention
This invention pertains to a superconducting element using a high-temperature oxide superconductor and more particularly relates to a superconducting element ideal as a Josephson junction element which is utilized as a memory element, a logical operator, a SQUID, etc.
2. Description of the Related Art
As Josephson junctions using a high-temperature oxide superconductor, such trilayer laminate type constructions as a superconducting thin film/insulating thin film/superconducting thin film structure (S/I/S construction) and a superconducting thin film/normal conducting thin film/superconducting thin film structure (S/N/S construction) have been studied. For the purpose of utilizing a Josephson junction as a switching element or a memory element and also for the purpose of deriving a large output voltage from a Josephson junction element, it is desirable to obtain an ideal S/I/S tunnel type junction. It has been here-tofore difficult, however, to obtain with high repeatability the S/I/S tunnel type junctions using a high-temperature oxide superconductor. For the purpose of obtaining with high repeatability a S/I/S tunnel type junction using a high-temperature oxide superconductor, it is necessary in the first place that the order parameter of superconductivity should be amply developed as far as the interface between the superconducting layer and the insulating layer. It is also necessary in the second place that the insulating layer approximately 2 nm in thickness should be formed without generating a short circuit or a leak of current.
For example, the S/N/S junction which uses a c-axis oriented Bi.sub.2 Sr.sub.2 Ca.sub.2 Cu.sub.3 O.sub.7 thin film as a superconducting layer and also uses a Bi.sub.2 Sr.sub.2 CuO.sub.y thin film manifesting no superconductivity as a normal conducting layer has been known. The trilayer laminate type Junction of this kind, however, has the drawback of giving rise to a metamorphic layer in the interfaces of lamination and failing to control this adverse nature. It is further at a disadvantage in that this Junction is liable to produce pinholes in the intermediate N layer and, at the same time, the N layer is easily converted into a superconductor by a change in the carrier concentration or composition. Owing to these drawbacks, the S/N/S junction of the structure mentioned above entails a problem of poor repeatability of the manifestation of Josephson characteristics.
Similarly, the S/N/S junction which has a Bi.sub.2 Sr.sub.2 CuO.sub.y thin film manifesting no superconductivity interposed between two c-axis oriented Bi.sub.2 Sr.sub.2 CaCu.sub.2 O.sub.y superconducting layers is reported by K. Mizuo et al. in Appl. Phys. Lett., Vol. 56, No. 15, pp. 1469 to 1471. Besides, the S/I/S junction which has a long-periodic layered copper oxide compound like Bi.sub.2 Sr.sub.2 (Ca,Sr,Bi).sub.7 Cu.sub.8 O.sub.20 interposed as an insulating layer between two c-axis oriented Bi.sub.2 Sr.sub.2 CaCu.sub.2 O.sub.y superconducting layers is reported by M. E. Klausmeier-Brown et al. in Appl. Phys. Lett., Vol. 60, No. 22, pp. 2806 to 2808. These trilayer junctions share the disadvantage that Ca and Sr diffuse from the superconducting layer into the intermediate layer of N layer or I layer, convert part of the intermediate layer locally into a superconductor, and tend to induce a leak of super current.
An attempt has been made to fabricate a S/I/S junction by forming a thin film of a simple oxide like ZrO.sub.2 or MgO or a thin film of a fluoride as an insulating layer by the sputtering technique or vacuum deposition technique. This trilayer junction, however, has a problem of pinholes in the insulating layer and of unduly low T.sub.c because the oxide superconducting layer is not easily deposited with a fine quality as film. JP-A-2-185,077, for example, discloses a Josephson junction element using fluorite (CaF.sub.2) for an insulating layer. This element is not considered to grow the insulating layer in such a manner as to allow lattice matching with the oxide superconductor. This element, therefore, is at a disadvantage in that the superconducting characteristics in and near the interfaces between the upper and lower oxide superconductors and the intervening insulating layer are deteriorated and, moreover, these interfaces are liable to form steps and pinholes. Particularly, this element has the disadvantage that fluorine diffuses in the upper and lower oxide superconducting layers and impairs the superconducting characteristics. JP-A-3-296,283 discloses a Josephson junction which has an insulating layer of an oxide represented by RE.sub.2 O.sub.w like PrO.sub.2 or Y.sub.2 O.sub.3 interposed between a pair of REBa.sub.2 Cu.sub.3 O.sub.7-x (wherein RE stands for a rare earth element) superconducting layers. This junction has the disadvantage that the insulating layer is liable to form steps and pinholes because it is not presumed to grow the insulating layer in a layer-by-layer growth.
Further, an attempt has been made to use such Perovskite type oxides as SrTiO.sub.3, PrGaO.sub.3, NdGaO.sub.3, LaSrGaO.sub.4, and BaTiO.sub.3 for an insulating layer. When such a simple insulating oxide as mentioned above and a copper oxide based superconductor are subjected to heteroepitaxial growth, the following drawbacks are intrinsically encountered. When a copper oxide based superconductor is grown in the form of a thin film, the growth unit is from a block layer to a block layer which both take no part in conduction. In the case of the oxide of Bi.sub.2 Sr.sub.2 CaCu.sub.2 O.sub.y, for example, the growth unit is from the Bi-O layer to the Bi-O layer. Then, in the case of the oxide of YBa.sub.2 Cu.sub.3 O.sub.7, the growth unit is from CuO.sub..delta. Chain to the CuO.sub.1-.delta. Chain. The surface which is formed in consequence of the growth of a lower superconducting electrode of an oxide superconductor consists of a block layer irresponsible for superconductivity. When an intermediate layer is superposed on the surface mentioned above and an upper superconducting electrode is then grown, it is likewise a block layer irresponsible for superconductivity that occurs directly above the intermediate layer. As a result, a normal conducting layer which is neither an insulator nor a superconductor is inevitably formed in the interface between the CuO.sub.2 layer responsible for superconductivity and an insulating oxide (intermediate layer). The attempt at fabricating a S/I/S junction in this case, therefore, is at a disadvantage in intrinsically giving rise to a S/N/I/N/S junction. This attempt, accordingly, fails to obtain a S/I/S junction which possesses ideal characteristic properties such that the order parameter of superconductivity is fully developed as far as the boundary of an insulating layer.
As a Josephson junction of trilayer laminate structure by epitaxial growth, the junction of a YBa.sub.2 Cu.sub.3 O.sub.7 /PrBa.sub.2 Cu.sub.3 O.sub.7-.delta. /YBa.sub.2 Cu.sub.3 O.sub.7 structure is being trially fabricated. This case is reported by J. B. Barner et al. in Appl. Phys. Lett., Vol. 59, No. 6, pp. 742 to 744. This junction has the disadvantage that the intermediate layer is easily converted into a conductor or a superconductor by mutual diffusion of Y and Pr.
Further, JP-A-4-105,373 discloses a superconducting element in which CuO.sub.5 pyramids are opposed to each other through the medium of a single fluorite-structural block (Ln.sub.2 O.sub.2) and a substance adjoining the apexes of the pyramids and having alternately superposed a part formed of an alkaline earth element and a part formed of oxides of Pb and Cu is used for an N layer or an I layer. This superconducting element uses the compound which contains a single fluorite-structural block. For the purpose of enabling this compound to function as an N layer or an I layer, therefore, the intermediate layer must be given a thickness of not less than two unit cells (typically a thickness of 50 nm). The compound containing the single fluorite-structural block is easily converted into a superconductor by a change in composition or by diffusion of an element from the superconducting layer. The thickness of the single fluorite-structural block measured as the distance between the CuO.sub.2 planes is approximately 0.6 nm, a value not appreciably large as compared with the coherence length in the direction of the C axis. The superconducting element under discussion, therefore, is at a disadvantage in respect that when this compound is converted into a superconductor, a superconducting linkage occurs also in the direction of C axis and the leak of current occurs readily.
Successful fabrication of a S/N/S Josephson junction of fine quality with high repeatability by the use of grain boundaries of a high-temperature oxide superconductor is reported by D. Dimos, P. Chaudhari, and J. Mannhart in Physical Review B., Vol. 41, No. 7, pp. 4038 to 4049 (1990). A similar Josephson junction is reported by P. Gross et al. in Supercond. Sci. Technol., Vol. 4, pp. S253 to 255 (1991). These junctions make use of the grain boundaries of a YBa.sub.2 Cu.sub.3 O.sub.7 thin film which is formed on the twin boundaries of a bicrystal substrate.
These grain-boundary Josephson junctions, however, entail the following drawbacks. Firstly, since the bicrystal substrate is fabricated by a method such as joining two crystals face to face, the process for preparing such substrates is complicated. Secondly, since the junctions are formed exclusively on the twin boundaries of the bicrystal substrate, they cannot be formed at a desired place on the substrates. Thirdly, since the Junctions are obtained exclusively in a planar pattern, laminate type junctions which allow a superconducting current to flow in a direction perpendicular to the surfaces of substrates cannot be fabricated. Fourthly, since the Josephson junctions are weak link junctions, they cannot be given as large a I.sub.c R.sub.n product as is normally possessed by a S/I/S junction.
As described above, the conventional laminate type Josephson junctions have the disadvantage of readily inducing the adverse phenomenon of current leakage because they have insufficiently matched interfaces between the oxide superconducting layers and the intermediate layers and they are liable to sustain such defects as steps and pinholes in the intermediate layers. They are further at a disadvantage in suffering the intermediate layers to be easily converted into conductors or superconductors as by a change in composition. Owing to these drawbacks, the conventional laminate type Josephson junctions have been unable to acquire ideal Josephson characteristics with satisfactory repeatability. In contrast, the grain-boundary Josephson junctions, though superior to the reported laminate type junctions in terms of characteristic properties, suffer from such drawbacks as complexity of the process of fabrication, many restrictions imposed on the manner and location of the formation, and inferior practical utility.
In the circumstances, an earnest desire has been expressed to perfect a laminate type Josephson junction which is vested with high practical utility and enabled to manifest improved Josephson characteristics with enhanced repeatability by eliminating such drawbacks on structure and material as mentioned above.